Management of voltage standing wave ratio at skin surface during microwave ablation

ABSTRACT

A dielectric spacer for use during microwave ablation of tissue is disclosed. The dielectric spacer includes a housing having a predetermined thickness and a skin-contacting bottom surface. The housing is configured to be filled with a dielectric material having a predetermined dielectric permittivity. The housing is further configured to be placed on the tissue in proximity with at least one microwave antenna assembly, wherein the thickness and the dielectric permittivity are configured to shift a maximum voltage standing wave ratio of the at least one microwave antenna assembly.

BACKGROUND

1. Technical Field

The present disclosure relates generally to microwave antennas. Moreparticularly, the present disclosure is directed to systems and methodsfor shifting voltage standing wave ratio at the tissue surface to reducethe amount microwave energy being deposited at the surface.

2. Background of Related Art

Treatment of certain diseases requires destruction of malignant tissuegrowths (e.g., tumors). It is known that tumor cells denature atelevated temperatures that are slightly lower than temperaturesinjurious to surrounding healthy cells. Therefore, known treatmentmethods, such as hyperthermia therapy, heat tumor cells to temperaturesabove 41° C., while maintaining adjacent healthy cells at lowertemperatures to avoid irreversible cell damage. Such methods involveapplying electromagnetic radiation to heat tissue and include ablationand coagulation of tissue. In particular, microwave energy is used tocoagulate and/or ablate tissue to denature or kill the cancerous cells.

Microwave energy is applied via microwave ablation antennas thatpenetrate tissue to reach tumors. There are several types of microwaveantennas, such as monopole and dipole, in which microwave energyradiates perpendicularly from the axis of the conductor. A monopoleantenna includes a single, elongated microwave conductor whereas adipole antenna includes two conductors. In a dipole antenna, theconductors may be in a coaxial configuration including an innerconductor and an outer conductor separated by a dielectric portion. Morespecifically, dipole microwave antennas may have a long, thin innerconductor that extends along a longitudinal axis of the antenna and issurrounded by an outer conductor. In certain variations, a portion orportions of the outer conductor may be selectively removed to providemore effective outward radiation of energy. This type of microwaveantenna construction is typically referred to as a “leaky waveguide” or“leaky coaxial” antenna.

Conventional microwave antennas operate at a single frequency allowingfor creation of similarly shaped lesions (e.g., spherical, oblong,etc.). Some antennas are capable of radiating energy inside as well asoutside tissue, due to well-tuned impedance matching. In some instancesthis may result in inadvertent radiation at the tissue surface.

SUMMARY

According to one embodiment of the present disclosure, a dielectricspacer for use during microwave ablation of tissue is disclosed. Thedielectric spacer includes a housing having a predetermined thicknessand a skin-contacting bottom surface. The housing is configured to befilled with a dielectric material having a predetermined dielectricpermittivity. The housing is further configured to be placed on thetissue in proximity with at least one microwave antenna assembly,wherein the thickness and the dielectric permittivity are configured toshift a maximum voltage standing wave ratio of the at least onemicrowave antenna assembly.

According to another embodiment of the present disclosure, a dielectricspacer for use during microwave ablation of tissue is disclosed. Thedielectric spacer includes a gel layer formed from a dielectric,elastic, shape-memory gel. The dielectric spacer also includes first andsecond substrates selected from at least one of film and foam and formedfrom a dielectric polymer. The first substrate is disposed on a topsurface of the gel layer and the second substrate is disposed on abottom surface of the gel layer, wherein the gel layer and the first andsecond substrates are configured to be perforated by at least oneantenna assembly.

According to a further embodiment of the present disclosure a dielectricspacer for use during microwave ablation of tissue is disclosed. Thedielectric spacer includes an inflatable, conformable housing having askin-contacting bottom surface and configured to be inflated to apredetermined thickness. The housing is configured to be filled with adielectric material having a predetermined dielectric permittivity. Thehousing is further configured to be placed on the tissue in proximitywith at least one microwave antenna assembly, wherein the thickness andthe dielectric permittivity are configured to shift a maximum voltagestanding wave ratio of the at least one microwave antenna assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will become more apparent in light of the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram of an ablation system according to thepresent disclosure;

FIG. 2 is a cross-sectional view of an antenna assembly of FIG. 1showing specific absorption rate with an unshifted voltage standing waveratio according to the present disclosure;

FIG. 3 is a cross-sectional view of an antenna assembly of FIG. 1showing specific absorption rate with a shifted voltage standing waveratio according to the present disclosure;

FIG. 4 is a perspective, cross-sectional view of a dielectric spaceraccording to one embodiment of the present disclosure;

FIG. 5 is a perspective, cross-sectional view of a dielectric spaceraccording to another embodiment the present disclosure;

FIG. 6 is a side, cross-sectional view of a dielectric spacer accordingto a further embodiment the present disclosure;

FIG. 7 is a perspective, cross-sectional view of a multi-probedielectric spacer according to one embodiment the present disclosure;

FIG. 8 is a perspective, cross-sectional view of a multi-probedielectric spacer according to another embodiment the presentdisclosure;

FIG. 9 is a perspective view of a dielectric insert according to oneembodiment the present disclosure;

FIG. 10 is a perspective view of a dielectric insert according toanother embodiment the present disclosure;

FIG. 11 is a perspective view of a dielectric spacer according to oneembodiment the present disclosure;

FIG. 12A is a perspective view of a dielectric spacer apparatusaccording to an embodiment of the present disclosure; and

FIG. 12B is a side, cross-sectional view of the dielectric spacer ofFIG. 12A;

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail to avoid obscuring the present disclosure in unnecessary detail.

FIG. 1 shows a microwave ablation system 10 that includes a microwaveantenna assembly 12 coupled to a microwave generator 14 via a flexiblecoaxial cable 16. The generator 14 is configured to provide microwaveenergy at an operational frequency from about 500 MHz to about 10,000MHz. In the illustrated embodiment, the antenna assembly 12 includes aradiating section 18 connected by feedline 20 (or shaft) to the cable16. The feedline 20 may be connected to a hub 22, which is connected tothe cable 16 through a cable connector 19. The hub 22 may have a varietyof suitable shapes, e.g., cylindrical, rectangular, etc.

The feedline 20 may be coaxial and may include an inner conductorsurrounded by an inner insulator, which is, in turn, surrounded by anouter conductor (e.g., a cylindrical conducting sheath). The inner andouter conductors are coupled to the cable 16 via the connector 19. Theinner conductor and outer conductor may be constructed of copper, gold,stainless steel or other conductive metals with similar conductivityvalues. The metals may be plated with other materials, e.g., otherconductive materials, to improve their properties, e.g., to improveconductivity or decrease energy loss, etc. In one embodiment, thefeedline 20 may be formed from a coaxial, semi-rigid or flexible cablehaving a wire with a 0.047″ outer diameter rated for 50 Ohms.

The connection hub 22 also couples the antenna assembly 12 to a coolingsystem 13. The connection hub 22 includes an outlet fluid port 30 and aninlet fluid port 32 that are connected in fluid communication with asheath 38. The sheath 38 encloses radiating portion 18 and feedline 20allowing a coolant fluid 37 to circulate from ports 30 and 32 around theantenna assembly 12. The ports 30 and 32 are also coupled to a supplypump 34 that is, in turn, coupled to a supply tank 36 via supply lines86 and 88, respectively. The supply pump 34 may be a peristaltic pump orany other suitable type. The supply tank 36 stores the coolant fluid 37and, in one embodiment, may maintain the fluid at a predeterminedtemperature. More specifically, the supply tank 36 may include a coolantunit that cools the returning liquid from the antenna assembly 12. Inanother embodiment, the coolant fluid 37 may be a gas and/or a mixtureof liquid and gas. The coolant fluid 37 provides for dielectricimpedance buffering for the antenna assembly 12. This allows forradiation of significant amounts of power while the antenna assembly 12is partially inserted in the tissue or exposed to air.

During operation, one or more of the antenna assemblies 12 are insertedinto tissue through the skin. In a percutaneous application, theinterface between the air and the tissue at the insertion pointdetermines the locations of voltage standing wave ratio (“VSWR”)maximums along the transmission path. A VSWR maximum forms at theinsertion point due to the dramatic change in impedance between air andtissue, significantly increasing the specific absorption rate (“SAR”)when compared with VSWR minimums. FIG. 2 illustrates the SAR profile ofthe antenna assembly 12 being inserted in the tissue “T” having a VSWRmaximum at the insertion point. This results in the increased radiationof microwave energy at the insertion point as illustrated by theablation volume “V.”

The present disclosure provides for a system and method of shifting theVSWR to effectively match the impedance of the tissue “T” on the surfacethereof. This involves shifting the location of the VSWR maximum nearestthe tissue surface into a material placed in proximity thereof. Morespecifically, the present disclosure provides for a dielectric spacer 90(FIG. 4) formed from a material that matches the impedance of the tissue“T” allowing for the shifting of the VSWR and thereby spreading out theSAR profile as shown in FIG. 3. More specifically, the SAR is minimizedat the surface of the tissue “T” due to the shift in VSWR maximum intothe spacer 90. In addition, the VSWR minimum is shifted to the interfacebetween the spacer 90 and the tissue “T,” thereby minimizing the effectsof microwave energy at the surface.

FIG. 4 illustrates the dielectric spacer 90 having a substantiallytoroidal shape. The spacer 90 may be formed from a solid dielectricmaterial having a dielectric permittivity from about 2 ∈_(r) to about 80∈_(r). The disclosed dielectric permittivity values are provide oneillustrative embodiment and are dependant on the dimensions of thematerial (e.g., dielectric spacer 90) as well as the frequency of theenergy supplied thereto and are thickness. The lower end of dielectricmaterials may include plastics, polymers (e.g., PTFE), and combinationthereof. Upper end of dielectric material may include ceramics. In oneembodiment, materials of various dielectric properties may be mixed(e.g., ceramic particles embedded in polymer gels). In anotherembodiment, the spacer 90 may include a rigid housing 92. The housing 92has a predetermined thickness “d” that is selected based on thefrequency of the microwave energy being supplied by the antennaassemblies 12 as well as the dielectric permittivity of the dielectricmaterial of the spacer 90. The distance “d” is selected to shift theVSWR by the predetermined amount, such that the minimum VSWR is at thesurface of the tissue “T.”

The housing 92 may be filled with any suitable dielectric liquid ordeformable material such as water, saline, dielectric gels, powders,etc. and mixtures thereof. If a dielectric liquid is used, thedielectric may be the coolant fluid 37. More specifically, the spacer 90may be coupled to the cooling system 13 via supply lines 97 and 99,which may be directly coupled to the pump or may be integrated with thesupply lines 86 and 88 of the antenna assembly 12.

In one embodiment, the spacer 90 may include one or more skintemperature monitoring devices 93, such as thermal probes,thermocouples, thermistors, optical fibers and the like, to monitor skinsurface temperature. The temperature monitoring devices 93 are coupledto the cooling system 13 and provide the temperature measurements orindicators thereto. The cooling system 13 then controls the flow and thetemperature (e.g., cooling) of the coolant fluid 37 based on thetemperature measured at the surface of the tissue “T.” In other words,the cooling system 13 increases the flow of the coolant fluid 37 and/ordecreases the temperature of the coolant fluid 37 when the temperatureof the tissue “T” increases and vice versa.

The spacer 90 also includes an aperture 91 defined therein for insertionof the antenna assembly 12 therethrough and into the tissue “T.” Theaperture 91 may be sized to be in frictional contact with the antennaassembly 12, thereby preventing movement of the antenna assembly 12while allowing for relatively easier insertion therethrough.

The spacer 90 may also include one or more fastening elements 98disposed on a skin-contacting bottom surface 94. The elements 98 may behooks, barbs and other tissue-penetrating elements suitable forretaining the spacer 90. The spacer 90 may also include an adhesivelayer 95 disposed on the bottom surface 94 thereof. In one embodiment, aprotective film may be disposed over the adhesive layer 95 to protectthe adhesive prior to use.

FIG. 5 shows another embodiment of a dielectric spacer 100 which isshaped as a bolus having a flexible expandable housing 102. The housing102 includes an aperture 101 defined therein for insertion of theantenna assembly 12 therethrough and into the tissue “T.” The aperture101 may be sized to be in frictional contact with the antenna assembly12 thereby preventing movement of the antenna assembly 12 while allowingfor relatively easier insertion. The housing 102 may be filled with anysuitable dielectric liquid or deformable material such as water, saline,dielectric gels, and mixtures thereof. If a dielectric liquid isutilized, the housing 102 may be coupled to the coolant system 13 toprovide for circulation of the coolant fluid 37 and may also include oneor more temperature monitoring devices (not explicitly shown) to providefor temperature-based cooling thereof as described above with respect tothe spacer 90.

In one embodiment, the spacer 100 may include one or more skintemperature monitoring devices 103 disposed internally or externally,such as thermal probes, thermocouples, thermistors, optical fibers andthe like, to monitor skin surface temperature. The temperaturemonitoring devices 103 are coupled to the cooling system 13 and providethe temperature measurements or indicators thereto. The cooling system13 then controls the flow and the temperature (e.g., cooling) of thecoolant fluid 37 based on the temperature measured at the surface of thetissue “T.” In other words, the cooling system 13 increases the flow ofthe coolant fluid 37 and/or decreases the temperature of the coolantfluid 37 when the temperature of the tissue “T” increases and viceversa.

The flexible expandable structure of the housing 102 allows the spacer100 to conform to the tissue. The conforming nature of the spacer 100allows for the inflation of the spacer 100 to desired dimensions,namely, to a predetermined thickness “d” that is selected based on thefrequency of the microwave energy being supplied by the antennaassemblies 12. The distance “d” is selected to shift the VSWR by thepredetermined amount, such that the minimum VSWR is at the surface ofthe tissue “T.” In addition, the conforming nature of the housing 102allows the aperture 101 to be shifted within the spacer 100 to providefor various angles of insertion of the antenna assembly 12 therethrough.

In another embodiment, the spacer 100 may be used with multiple probes,which obviates the need for the aperture 101. The spacer 100 may beplaced in proximity of (e.g., between) a plurality of antenna assemblies12 as shown in FIG. 6. Ablations involving multiple probes, namely,simultaneous application of microwave energy through a plurality ofantenna assemblies 12 provide additional risk of VSWR occurrence. Inother words, a VSWR maximum occurs at the tissue-air boundary around andbetween the antenna assemblies 12, which has the potential to cause skinburns in high-power and/or long-duration microwave applications.Ablations using a plurality of antenna assemblies 12 are particularlysusceptible to causing high power absorption rates near the surface dueto the potential for so-called “two-wire” line energy propagation fromthe radiating sections 18. Thus, placement of the spacer 100 between theplurality of antenna assemblies 12 shifts the VSWR maximum away from thesurface of the tissue “T” as shown in FIG. 3 and described above withrespect thereto.

FIG. 7 shows another embodiment of a dielectric spacer 110 having asubstantially toroidal shape. The dielectric spacer 110 may also have agranular (e.g., triangular) shape as shown in FIG. 8. The spacer 110 maybe formed from a solid dielectric material having a dielectricpermittivity from about 2 ∈_(r) to about 80 ∈_(r). In anotherembodiment, the spacer 110 may include a rigid housing 112. The housing112 has a predetermined thickness “d” that is selected based on thefrequency of the microwave energy being supplied by the antennaassemblies 12. The distance “d” is selected to shift the VSWR by thepredetermined amount, such that the minimum VSWR is at the surface ofthe tissue “T.”

The housing 112 may be filled with any suitable dielectric liquid ordeformable material such as water, saline, dielectric gels, and mixturesthereof. If a dielectric liquid is utilized, the housing 112 may becoupled to the coolant system 13 to provide for circulation of thecoolant fluid 37 and may also include one or more temperature monitoringdevices to provide for temperature-based cooling thereof as describedabove with respect to the spacer 90.

The spacer 110 includes two or more apertures 111 defined therethroughfor insertion of the antenna assemblies 12 and into the tissue “T.” Inone embodiment, any number of apertures 111 may be disposed within thespacer 110 to accommodate a desired number of the antenna assemblies 12.The apertures 111 may be sized to be in frictional contact with theantenna assemblies 12 thereby preventing movement of the antennaassemblies 12 while allowing for relatively easier insertion.

The spacer 110 may also include one or more fastening elements 118disposed on a skin-contacting bottom surface 114. The elements 118 maybe hooks, barbs and other tissue-penetrating elements suitable forretaining the spacer 110. The spacer 110 may also include an adhesivelayer 115 disposed on the bottom surface 114 thereof. In one embodiment,a protective film may be disposed over the adhesive layer 115 to protectthe adhesive prior to use.

The spacer 110 also includes a centrally disposed chamber 116 having asubstantially cylindrical shape and a removable dielectric insert 117 asshown in FIG. 9. The apertures 111 are disposed radially around thechamber 116. The dielectric insert 117 also has a substantiallycylindrical shape suitable for insertion into the chamber 116 and may besized to be in frictional contact with the chamber 116. In anotherembodiment, the chamber 116 and the dielectric insert 117 may be of anysuitable symmetrical shape configured to shift the VSWR.

The dielectric insert 117 may be formed from a solid dielectric materialhaving a dielectric permittivity from about 2 ∈_(r) to about 80 ∈_(r).The removable insert 117 configuration allows for use of various inserts117 to provide for a suitable impedance match with the tissue “T.” Theinsert 117 may be specifically designed for use with specific tissuetypes (e.g., liver, lung, kidney, bone, etc.) as well as the operationalfrequency of the generator 14.

FIG. 10 shows another embodiment of a removable dielectric insert 118having two or more stackable subassemblies 119 and 120. The firstsubassembly 119 is formed from a dielectric material having a firstdielectric permittivity from about 2 ∈_(r) to about 80 ∈_(r). The secondsubassembly 120 is formed from a dielectric material having a seconddielectric permittivity from about 2 ∈_(r) to about 80 ∈_(r). The firstand second subassembly 119 and 120 may frictionally fit together (e.g.,female and male connectors) to form the removable dielectric insert 118.The insert 118 provides additional flexibility in selecting specificmaterial to find the ideal impedance match with the tissue “T” to shiftthe VSWR.

FIG. 11 shows another embodiment of a dielectric spacer 140 having asubstantially disk-like shape. The spacer 140 has a multi-layeredstructure and includes a gel layer 142 disposed between a top substrate144 and a bottom substrate 146 (e.g., the top substrate 144 disposed ona top surface of the gel layer 142 and the bottom substrate 146 disposedon a bottom surface of the gel layer 142). The gel layer 142 may beformed from a dielectric, elastic, shape-memory gel, such as hydrogelsor adhesives or other polymer-based materials having a dielectricpermittivity from about 2 ∈_(r) to about 30 ∈_(r). The dielectricproperties of the gel layer 142 shift the VSWR thereby avoiding heatdamage to the surface of the tissue “T” as shown in FIG. 3.

The substrates 144 and 146 may be formed from any type of dielectricpolymer-based material, such as polyurethane. The substrates 144 and 146may be formed as a film or a foam suitable for penetration by theantennas assemblies 12. In one embodiment, the substrates 144 and 146may be formed from thermal paper for indicating changes in thetemperature of the tissue “T.” The spacer 140 may also include anadhesive layer 145 disposed on a skin-contact bottom surface 147 of thebottom substrate 146. In one embodiment, a protective film may bedisposed over the adhesive layer 145 to protect the adhesive prior touse.

In one embodiment, the substrates 144 and 146 and the gel layer 142 mayhave one or more openings (not explicitly shown) defined therein tofacilitate insertion of the antenna assemblies 12. In anotherembodiment, the substrates 144 and 146 and the gel layer 142 may becontiguous such that the antenna assemblies 12 perforate the multiplelayers during insertion. The gel and/or adhesives of the gel layer 142maintains the antenna assemblies 12 at the desired depth therebypreventing displacement thereof during ablation.

FIGS. 12A-12B show a dielectric spacer 220 for shifting the VSWR of anantenna assembly 222 within the tissue “T” (FIG. 12A). The dielectricspacer 220 may be formed from a dielectric, elastic, shape-memory gel,such as hydrogels or adhesives or other polymer-based materials having adielectric permittivity from about 2 ∈_(r) to about 80 ∈_(r). In oneembodiment, the dielectric spacer 220 may be formed from variousparticles (e.g., polymer, ceramic, etc.) mixed with the hydrogels oradhesives. The dielectric spacer 220 may be utilized as a formed (e.g.,gel layer 142) or unformed gel layer (as shown) for insertion of theantenna assembly 222 therethrough.

In one embodiment, the dielectric spacer 220 may be formed from anadhesive amorphous putty that may be molded under pressure but is stillcapable of retaining its shape. In other words, the putty may be shapedfrom a first configuration into a subsequent configuration for securingthe antenna assembly 222 therein. In one embodiment, the amorphous puttymay be a viscoelastic polymer composition having a siloxane polymer, acrystalline material and one or more thixotropic agents to reduce liquidproperties thereof and enable the amorphous putty to hold its shape.

During use, the dielectric spacer 220 is placed onto the tissue “T” andthe antenna assembly 222 is inserted therethrough perforating thedielectric spacer 200. The viscoelastic properties of the spacer 220allow the antenna assembly 222 to easily penetrate therethrough and intothe tissue “T” as shown in FIG. 12B. Since the putty of the spacer 220is adhesive, the putty secures the spacer 220 to the tissue “T” andmaintains the position of the antenna assembly 222 therein. Theabove-discussed embodiments of dielectric spacers may also be utilizedwith a spreadable dielectric gel layer (e.g., ultrasound gel, petroleumgel, etc.) to provide for additional VSWR shifting.

The described embodiments of the present disclosure are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present disclosure. Various modifications andvariations can be made without departing from the spirit or scope of thedisclosure as set forth in the following claims both literally and inequivalents recognized in law.

1-17. (canceled)
 18. A dielectric spacer for a microwave ablationantenna, the dielectric spacer comprising: a conformable housing havinga tissue-contacting surface, the housing including: a lumen configuredfor insertion of a microwave antenna assembly therethrough; and a cavitydisposed around the lumen, the cavity being configured to be filled witha dielectric material to expand the housing.
 19. The dielectric spaceraccording to claim 18, wherein the dielectric material is a coolant. 20.The dielectric spacer according to claim 19, wherein the coolant isselected from the group consisting of water and saline.
 21. Thedielectric spacer according to claim 20, wherein the housing is coupledto a cooling system configured to circulate the coolant through thecavity.
 22. The dielectric spacer according to claim 21, furthercomprising at least one temperature monitoring device configured tomeasure a temperature of at least one of the tissue or the housing. 23.The dielectric spacer according to claim 22, wherein the cooling systemis configured to adjust at least one of a flow rate or a temperature ofthe coolant based on the measured temperature by the at least onetemperature monitoring device.
 24. A system for use during microwaveablation of tissue, the system comprising: a dielectric spacerincluding: a conformable housing having a tissue-contacting surface, thehousing including: a lumen configured for insertion of a microwaveantenna assembly therethrough; and a cavity disposed around the lumen,the cavity being configured to be filled with a dielectric material toexpand the housing; and a cooling assembly configured to circulate thedielectric material through the cavity.
 25. The system according toclaim 24, wherein the dielectric material is a coolant.
 26. The systemaccording to claim 25, wherein the coolant is selected from the groupconsisting of water and saline.
 27. The system according to claim 26,further comprising at least one temperature monitoring device configuredto measure a temperature of at least one of the tissue or the housing.28. The system according to claim 27, wherein the cooling assemblyadjusts at least one of a flow rate or a temperature of the coolantbased on the measured temperature by the at least one temperaturemonitoring device.
 29. A method for microwave ablation comprising:inserting a microwave antenna into a lumen defined by a housing of adielectric spacer, the dielectric spacer having a cavity disposed aroundthe lumen; supplying a dielectric material into the cavity, thedielectric material having a predetermined dielectric permittivity; andexpanding the housing to adjust at least one dimension of the housingand a voltage standing wave ratio of the microwave antenna.
 30. Themethod according to claim 29, wherein supplying the dielectric materialinto the cavity includes circulating the dielectric material through thecavity through a cooling assembly.
 31. The method according to claim 30,measuring a temperature of at least one of the tissue or the housing.32. The system according to claim 31, adjusting at least one of a flowrate or a temperature of the dielectric material based on the measuredtemperature by the at least one temperature monitoring device.
 33. Themethod according to claim 29, wherein the dielectric material is acoolant.
 34. The method according to claim 33, wherein the coolant isselected from the group consisting of water and saline.